1. Field of the Invention
This invention relates to systems for simulating an infrared image for use in testing infrared seekers. It also relates to detector arrays for electromagnetic radiation including infrared and microwave.
2. Description of the Related Art
It would be highly desirable to be able to simulate a real-time infrared (IR--roughly 0.7-20 microns wavelength) image that is substantially free of flicker. This would provide an effective way to test IR detectors, also referred to as "seekers" and "focal plane arrays". At present, problems of excessive flicker impose a serious constraint on IR simulation systems. A basic problem with image flicker is that it creates a false target indication, since flicker corresponds to a change in the temperature of the IR image. Unlike the human eye which integrates light flicker over a period of about 30-50 msec., IR detectors integrate flickers over periods of only about 1-5 msec. Thus, there is a significant range over which flicker (in the visible spectrum) would not be detected by the human eye but would be picked up by an IR detector if it is within the IR spectrum.
Excessive flicker has been avoided heretofore with the use of a Bly cell to project a static image that has been applied to the cell. Bly cells are described in Vincent T. Bly, "Passive Visible to Infrared Transducer for Dynamic Infrared Image Simulation", Optical Engineering, Nov./Dec. 1982, Vol. 21, No. 6, pp. 1079-1082. However, the requirement that this type of system be operated with a static image is a significant limitation, since a more meaningful test of IR detectors calls for the detection of images that can change in real-time.
A prior attempt to produce an IR simulation system with a real-time image involved the formation of a video image by a cathode ray tube (CRT). The CRT video image was applied as an input to a liquid crystal light valve (LCLV), to which an IR readout beam was applied. The LCLV modulated the IR readout beam with the video image from the CRT to produce a corresponding IR video image. See S. T. Wu et al, "Infrared Liquid Crystal Light Valve", Proceedings of the SPIE, Vol. 572, pages 94-101, August, 1985.
This approach unfortunately was found to result in a substantial amount of flicker. The problem is that the illuminated pixels on the CRT screen decay in intensity over time prior to the next electron beam scan. This causes an undesirable intensity gradient to appear on a projected IR image from an IR-LCLV which is coupled to the CRT, and an IR detector will then detect a non-uniform image. Because the detector is generally looking for intensity gradients, or edges, by which its associated algorithms determine the presence of "targets", such intensity gradients are misleading. While this problem could theoretically be solved by synchronizing the IR detector scan with the CRT electron beam scan, such synchronization may not be desired in many applications. Thus, although an IR-LCLV has the capability of projecting high resolution, high dynamic range, real-time simulated IR images when compared to a Bly cell, this advantage is mitigated by the CRT pixel decay. Furthermore, electrically driven matrix emitter devices have flicker if driven with simple RC-type pixel addressing circuits, since the RC decay is similar in effect to the phosphor decay of the CRT.
Modifications of the basic CRT-LCLV system described above might be envisioned to reduce or eliminate flicker, but they introduce other problems. In one such modification, two storage CRTs are provided with shutters in front of each screen. Operation is alternated between the two CRTs by means of the shutters, so that they are alternately applied to the LCLV. By staggering the video data frames between the two CRTs, the phosphor decay seen by the LCLV could theoretically be reduced significantly. However, in such a system, it may be difficult to implement the very fast shutter coordination that would be necessary to substantially avoid flicker. Furthermore, storage CRTs are non-uniform, resulting in image differences and consequent flicker.
Another approach would be to use a single CRT, but to increase the frame rate of the Raster scan from the conventional rate of about 30 Hz to a much higher rate, perhaps about 1,000 Hz. The CRTs of the future may provide higher bandwidths than that presently attainable, thereby making this approach more attractive.
A possible approach which does not provide real-time addressable images is the use of a "flicker-free" film or slide projector like the SCANAGON.RTM. device produced by Robert Woltz Associates, Inc. of Newport Beach, Calif. and disclosed in U.S. Pat. Nos. 4,113,367 and 4,126,386, or a comparable image projector. While the potential may exist for this limited technique, it has not been demonstrated to provide jitter-free and flicker-free images. Furthermore, this method will not provide real-time electronically updatable imagery.
In addition to flicker-free images, it is very important for an IR simulation system to achieve high spatial resolution, large dynamic temperature ranges and fast response. Spatial resolutions should be at least 500.times.500 pixels, with flicker less than 1%. For some applications frame rates should be 100 Hz or greater, and the dynamic thermal range should ideally be from near room temperature (some applications require cooled background temperatures) to 1,000.degree. C., particularly in the 3-5 micron spectral range. This combination of dynamic range and response time requirements is difficult for commercially available liquid crystals to achieve. Furthermore, most IR simulation applications require the simulator to be mounted on the two-axis target arm of a Carco table, which is then moved relative to the IR seeker being tested. For this purpose weight and size limitations are very important. The liquid crystal based IR simulator requires a large blackbody source, typically in excess of 100 pounds, and an expensive wire grid polarizer to be mounted with the active matrix. This reduces the mobility of the supporting Carco table arm.
Liquid crystal systems are also limited in dynamic range because it is difficult to photogenerate enough charge in the silicon substrate, while maintaining spatial resolution, to rotate the liquid crystal molecules completely. Furthermore, there is a restriction on the speed of response of liquid crystals if a reasonable thermal dynamic range is to be maintained. The flicker problem associated with liquid crystal systems can be reduced by utilizing a flicker-free visible addressing source for the liquid crystal light valve. However, known sources of this type are limited in speed, resolution, weight and size.
U.S. Pat. No. 4,724,356 to Daehler discloses a resistorbased IR simulator in which an array of resistors are individually addressed to stimulate IR emission. The Daehler approach is also discussed in Daehler, "Infrared Display Array", SPIE Vol. 765, Imaging Sensors and Displays (1987), pages 94-101 and Burriesci et al., "A Dynamic RAM Imaging Display Technology Utilizing Silicon Blackbody Emitters", SPIE Vol. 765, Imaging Sensors and Displays (1987), pages 112-122. Both the resistors and their respective drive transistor circuits are formed from a bulk silicon wafer. An air gap groove is formed under the resistors to help reduce thermal conduction losses to the underlying bulk silicon wafer. To facilitate the selective etching used to form the insulating air gap grooves, the top layer of the wafer is heavily doped. However, this doping makes the top layer electrically conductive, resulting in a significant impedance mismatch between the resistors and their respective transistor drive circuits. As a result, most of the input power is lost in the drive circuitry, rather than going into IR radiation. This reduction in the power transfer to the resistors results in a need for even more input power to the transistors, leading to a potential overheating problem.
In addition to an inefficient use of input power, in the Daehler approach only a relatively small portion of the pixel area (less than 10%) is actually occupied by the radiating resistor. Much of the area that might otherwise be devoted to the resistor is occupied by the drive transistors and the insulating air gap. As a result, to achieve a given apparent temperature for the pixel as a whole the resistor itself must be heated to a significantly higher temperature. This substantially reduces the system's effective thermal dynamic range.
Another problem with the Daehler approach relates to the electrical lead lines used to address the individual pixels. Relatively large silicon wafers must be used with present technology to achieve a significant number of pixels. For example, a wafer of about four inches is required for a 512.times.512 array. This means that the electrical lead lines on the chip must also be about four inches long. This in turn leads to two significant problems. First, there is a large amount of capacitive coupling associated with the long lead lines. Parasitic capacitance between conductive substrate and metal lead lines can limit the response time. Second, defects such as pin holes can appear in the oxide insulator formed on the substrate, causing a short circuit between the line and the silicon substrate which renders the associated pixel or pixels inoperative. The longer the lines, the greater is the probability of such defects. Daehler tries to resolve this problem by limiting the size of the array to 8.times.128 pixels, and coupling many such arrays together side-by-side to produce an aggregate array of reasonable size. However, the coupling process itself introduces significant complexities.
There is also a need for a two-dimensional IR detector array, and for detector arrays at other electromagnetic radiation (emr) wavelengths such as microwave, which have a fast response and make efficient use of available area.
Various types of IR thermal detectors are currently available. In pyroelectric detectors, a temperature change alters the dipole moment of the material, resulting in a charge difference between crystal faces. This type of detector is discussed in an article by Watton, "Ferroelectrics for Infrared Detection and Imaging", Proceedings of the Sixth IEEE International Symposium on the Application of Ferroelectrics, June 1986, Bethlehem, Pa. pages 172-181. Another type of IR detector is the bolometer, in which the energy of absorbed radiation raises the temperature of the detecting element to change its electrical resistance, which change is measured as an indication of the amount of received radiation. An array of such devices is disclosed in Boninsegni et al., "Low Temperature Bolometer Array", Review of Scientific Instruments, Vol. 60, No. 4, April 1989, pages 661-665. Both the pyroelectric and bolometer devices exhibit a relatively slow rate of response to changes in IR level, and also require relatively large areas. The pyroelectric thermal detector also suffers from limited resolvable temperature differences, low yield and high cost processing, a necessary hybrid integration and AC operation with a chopper. The limited temperature resolution, which is principally due to the inherent thermal conduction loss and large thermal mass, can be improved by thinning and reticulating the pyroelectric wafer, but this involves very difficult and expensive processing.
Another approach to IR detection involves an array of single crystal Schottky junctions, as disclosed in Tsaur et al., "IR Si Schottky-Barrier Infrared Detectors With 10-.mu.m Cutoff Wavelength", IEEE Electron Device Letters, Vol. 9, No. 12, December 1988, pages 650-653. This device is used as a photon detector rather than a thermal detector, has a very limited spectral range, and has to be operated at cryogenic temperatures.
Thermocouple devices provide another IR thermal detector. In this type of device junctions are formed between dissimilar materials, with a voltage induced across the junction in response to heating. Such a device is disclosed in Lahiji et al., "A Batch-Fabricated Silicon Thermopile Infrared Detector", IEEE Transactions on Electron Devices, Jan. 1982, pages 14-22. In this article a series of thermocouples have hot junctions which are supported on a thin silicon membrane by integrated circuit techniques. The membrane area is coated with a layer of IR absorbing material to efficiently absorb energy from the visible to the far infrared. The sensitivity of the device is limited because the thermocouples are formed directly on the substrate and therefore cannot heat as much as desired due to the thermal conduction losses. Also, a relatively large area is required for the detector.
Bolometers and crystal detectors have also been used to sense emr at microwave frequencies. Since relatively large sensor dimensions are necessary, single detectors have been used to scan an incoming image, rather than using a detector array to sense the entire image at one time. The result has been slow frame rates. The same problem has attended the use of single scanning junction detectors in the IR require.